Answer:
A. Sees
Step-by-step explanation:
A. "sees" states that it is third person, present tense and possibly plural (eg: The group sees a dog walk across the road).
B. "will see" states that it is either first, second, or third person, future tense, and either singular or plural (eg: You will see great results!).
C. "is seeing" is third person, present tense, but it is only singular (eg: She is seeing the toddler fall).
D. "have seen" is third person, past tense, and plural (eg: They have seen cases like this before).
Hope this helps! :)